7.1 SN2 Reactions

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    Anatomy of a Nucleophilic Substitution Reaction

    nucleophilic substitution reaction

    -Common leaving groups include Cl-, Br-, I- and OTs-

    SN2 Reactions - Substitution Nucleophilic Bimolecular

    sn2 back side attack
    sn2 transition state

    Bimolecular – Two reactant molecules are involved in the slow step and the rate law is 2nd order.

    Rate = k[substrate][nucleophile]

    -The substrate is often al alkyl halide (Cl, Br, and I are good leaving groups).

    -The mechanism is concerted and involves back-side attack.

    -SN2 reactions result in inversion of configuration at a chiral center (Walden inversion).

    sn2 reaction 3

    Substrate Steric Effects

    sn2 substrate steric effects

    Effect of Beta Branching

    sn2 beta branching

    Nucleophile Effects

    Require a strong nucleophile – the stronger the nucleophile the faster the reaction.

    Note: most strong nucleophiles are negatively charged.

    strong nucleophiles
    nucleophile strength in protic solvents
    nucleophile strength in aprotic solvents

    Solvent Effects

    -Negatively charged nucleophiles are stronger in polar aprotic solvents

    polar aprotic solvents

    Leaving Group Effects

    -Generally the weaker the base the better the leaving group; the better the leaving group the faster the reaction.

    sn2 leaving group

    Use of KF with crown ethers for SN2 reactions

    use of crown ether
    18-crown-6